The invention relates to multiple channel analog data acquisition systems in general, and more particularly to automatic calibration of a computerized system for temperature measurements.
Modern analog data acquisition systems use digital treatment with a computer centrally located and remote from local measuring points which are scanned or sequentially selected for initial testing and calibration concurrently with analog measurement.
Remote control and monitoring in a hostile evnironment involving high electrical noise and wide temperature variations require calibration measurement to insure the accuracy and performance expected for digital computer treatment and control with the derived information. Improved computer-based systems are desirable to be able to place high performance complex systems in very small packages. Microcomputer treatment has been further extended to the design and operation of the analog portion of the system, thus, right across the analog interface of the system. This has been done, in particular, for automatic calibration in multiple channel analog-to-digital data acquisition systems. See for instance U.S. Pat. Nos. 3,961,325; 3,975,727 and 4,016,557.
Calibration involves an initial correction and setting of the instrumentation to account for gain and offset errors, for inaccuracies associated with filters, current sources, thermocouple effects, and lead-lines inserted with the instrumentation.
One approach in the prior art when confronted with a computerized data acquisition system has been to merely reapply to digital systems the techniques already known for totally analog systems. This approach, however, requires signal substitution at the terminal points, or signal injection at some intermediary location in the system. In the first instance there is a need for disconnecting wires with the additional drawback of possible error in making the connections on the terminal block. In the second instance, test and calibration are effective only partially on the system. Short of automatic calibration, adjustments are made manually on a number of trimpots until the operator observes a predetermined level reached by the signal under consideration. Considering the large number of inputs in a multiplexed analog I/O subsystem, with a manual approach only the common hardware is compensated by calibration and a residual error remains. In order to minimize such residual error, the errors associated with each input may be called for and the calibration made on the common hardware be corrected with the average of the two most extreme detected errors. These procedures are tedious, costly, time consuming, and still they allow a residual error on most inputs.
A better approach than disconnecting wires consists in controlled switching, for testing purposes, of a special signal generator. This approach, however, also disconnects, if not manually, the main measurement signal paths. For increased reliability, this approach requires additional hardware to determine whether the measuring path has been properly reconfigured after calibration.
In contrast to prior art approaches, the present invention proposes in a multiplex data acquisition system to combine hardware and software in such a way that automatic signal substitution for calibration is provided without physically interrupting the measurement signal path of any of the selected channels.